1. Field of the Invention
The present invention relates to a smoke sensing chamber in a photoelectric smoke sensor having a large opening for easily introducing smoke thereinto, which permits preventing light from entering the chamber from the outside, preventing a decrease in SN ratio due to the reflection of the light beam from a luminous element provided in the chamber, and reducing the consumption of electric power of a detection circuit; and a smoke sensor box which can be combined with the smoke sensing chamber and which is of a simple construction permitting an accurate mounting of a luminous element and a light receiving element thereon.
2. Description of the Prior Art
A smoke sensing blind box used to photoelectrically detect smoke and give an alarm for a fire and consisting of a cylindrical body having inner and outer walls has been known. The cylindrical body is closed at both ends thereof and the slits in the inner walls and those in the outer wall are so arranged that the former are not in alignment with the latter.
In order to easily introduce smoke into a blind box, many slits may be provided in the walls thereof. However, a blind box with many slits in the walls thereof may allow the light to enter the same from the outside to cause an erroneous alarm. A structure having simple double walls may not give a solution to the problem of effecting at once the facilitating of introduction of smoke and the prevention of entry of light from the outside. This causes contradictory results.
When the number of slits in the outer and inner walls is increased while widening the gap between the outer and inner walls to increase the total area of the openings in the cylindrical body, the amount of smoke flowing thereinto may be surely increased but the incident angle of the light advancing into the blind box may be decreased. Then, even when the slits in the outer wall and the slits in the inner wall are so arranged that the former are not in alignment with the latter, much more light may enter the blind box. In order to eliminate the inconvenience, it is necessary that the dimensions of the blind box be increased limitlessly and that the sensitivity of the detection circuit be reduced beforehand to such an extent that corresponds to the amount of scattered light entering the blind box from the outside. Consequently, smoke cannot be detected at a high accuracy with a small consumption of electric power.
In a device for sensing smoke with a sensor provided in a smoke collecting chamber thereof, which smoke is introduced into the chamber when a fire occurs, a light receiving element senses irregularly reflected scattered light occurring when the light from a luminous element impinges upon the particles of smoke. A sensor portion including a luminous element and a light receiving element in such a device is generally provided in a groove formed in the bottom portion of the smoke collecting chamber, and the sensor and chamber are integrally molded. Therefore, it is troublesome to mount the luminous element and light receiving element in the groove. These elements may not be fixed in a predetermined position, and it is troublesome to adjust the position thereof. Thus, a large number of assembling steps are required.